This invention relates to the prevention of internal corrosion in a water injection well with galvanic sacrificial anodes. Preferably, the anodes will be capable of being installed and removed by a wireline.
In the production of crude oil and other minerals, water is frequently injected through a well into a subsurface formation. The wells may be converted producing wells or they may be specifically drilled for water injection. The typical water injection well has a casing set alone or extending past a subsurface formation into which the water is to be injected. In the latter case, the casing is perforated over an appropriate interval of the subsurface formation. The water is injected through tubing extending from the surface to a well packer set above the open end or perforated area of the casing. This leaves a section of casing below the packer. It is common practice to internally coat the tubing with a protective coating and to protect the external or outer surface of the tubing by sealing off the casing-tubing annulus with a packer with a treated fluid above the packer. On the other hand, the internal surface of the section of casing below the packer rarely has a protective coating. This internal surface of the casing corrodes unless the water is carefully treated and controlled or a corrosion inhibitor is injected with the water. The water flow rate may vary or even be shut off for certain periods. Corrosion inhibition and surface treatment of the injection water are the established ways of preventing internal casing corrosion of the section. Even if these standard practices were otherwise effective, metal anomalies may be present in the casing and cause corrosion.
It is common practice to use sacrificial anodes for prevention of corrosion of certain surface equipment like heaters and to use cathodic protection for protection of exterior surfaces. The weight and size of the anodes is based on the nature of the fluids or soils and the surface areas to be protected. Generally, there is adequate space for adequate size and weight anodes and for enough distance to allow the protective current to spread out. In addition, it is relatively easy to change the anodes when needed. In a water injection well, the space is very limited because the tubing must be adequately sized to carry sufficient rates of water injection and because the diameter of the casing is quite limited. The use of sacrificial anodes for protection of the internal surface of a lower section of casing in a water injection appears not to have been previously suggested.